fix(client): Check the user's old password before attempting a change password.

* Add 'checkPassword' to fxa-client.js
* Check the user's pasword before checking whether the passwords are the same.

fixes #1118
Change `assert.ok(false,` to `assert.fail`. Thanks @zaach!
Merge pull request #1290 from mozilla/issue-1118-change-password-errors

fix(client): Check the user's old password before attempting a change password.